1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular, to correction of aberration of an image position.
2. Related Background Art
For color image forming apparatuses, various systems have been proposed which have a plurality of image forming parts in order to increase processing speed and which sequentially transfer images of different colors onto a recording material held on a conveying belt.
Problems with such apparatuses having the plurality of image forming parts include color aberration (one of position aberration) in which when the color images are placed on one another, the color images are not overlapped at a common position due to an irregular movement of a plurality of photosensitive drums or the conveying belt associated with mechanical accuracy or the like or due to the variation, for each color, of a relationship between an outer peripheral surface of the photosensitive drum and the movement of the conveying belt in the transfer positions of the image forming parts.
In particular, in apparatuses including a plurality of image forming parts each having a laser scanner and a photosensitive drum, there are errors in the distance between the laser scanner and the photosensitive drum between the image forming parts. If these errors are different among the image forming parts, the laser scanning width on the photosensitive drum varies, resulting in color aberration.
FIGS. 2A, 2B, 2C and 2D show an example of a color aberration. Reference numeral 7 denotes an original image position, and reference numeral 8 (8a-8d) denotes an image position observed when the color aberration occurs. Additionally, FIGS. 2A, 2B and 2C show cases in which the color aberration occurs in a scanning direction. In those figures the two lines are separately drawn in a conveying direction for easy understanding.
FIG. 2A indicates an inclination of a scanning line which occurs when an optical part and the photosensitive drum are inclined relative to each other. This inclination can be corrected to the arrowed direction by, for example, adjusting the position of the optical part, the photosensitive drum, or a lens.
FIG. 2B indicates a scale aberration caused by variations in the distance between the optical part and the photosensitive drum. The case is likely to occur when the optical part comprises laser scanner. This aberration can be corrected to the arrowed direction by, for example, fine-tuning image frequency (increasing the frequency if the scanning width is large) to change the length of the scanning line.
FIG. 2C indicates a write start position error in the main-scanning direction. This error can be corrected to the arrowed direction by, for example, adjusting a write start timing with respect to a beam detected location if the optical part comprises a laser scanner.
FIG. 2D indicates a sub-scanning direction (sheet conveying direction) aberration. This can be corrected to the arrowed direction by the arrow by, for example, adjusting the write start timing with respect to detection of a leading end of a sheet for each color.
To correct these color aberrations, position aberration detecting patterns for corresponding colors are conventionally formed on the conveying belt and are detected by a pair of optical sensors provided on the respective sides of a downstream part of the conveying belt so that various adjustments as described above are carried out depending on the amount of aberration detected.
FIG. 3 shows an example of such position aberration detecting patterns. Reference numerals 9 and 10 denote patterns for detecting position aberration in the sub-scanning direction. Reference numerals 11 and 12 denote patterns for detecting position aberration in the scanning direction, which are inclined from a conveying direction of belt 3 through 45xc2x0 in FIG. 3. The patterns 9 to 11 have been transferred onto the conveying belt.
Reference numerals 9a, 10a, 11a, and 12a denote black patterns (hereafter referred to as xe2x80x9cBkxe2x80x9d), reference numerals 9b, 10b, 11b, and 12b denote yellow patterns (hereafter referred to as xe2x80x9cYxe2x80x9d), reference numerals 9c, 10c, 11c, and 12c denote magenta patterns (hereafter referred to as xe2x80x9cMxe2x80x9d), and reference numerals 9d, 10d, 11d, and 12d denote cyan patterns (hereafter referred to as xe2x80x9cCxe2x80x9d).
Reference numerals tsf1 to tsf4, tmf1 to tmf4, tsr1 to tsr4, and tmr1 to tmr4 denote timings with which sensors 6a and 6b detect the patterns. The arrow A denotes the direction in which a conveying belt 3 moves.
Here, the movement speed of the conveying belt 3 is defined as v mm/s, Bk is assumed to denote a reference color, and the theoretical distances between each of the color pattern for the sheet conveying direction and the Bk pattern is defined as dsY mm, dsM mm, and dsC mm. The measured distances between each of the color pattern for the sheet conveying direction and a corresponding one of the patterns for the scanning direction are defined as dmfBk mm, dmfY mm, dmfM mm, and dmfC mm on the left side and as dmrBk mm, dmrY mm, dmrM mm, and dmrC mm on the right side.
When the Bk is assumed to denote the reference color, aberration xcex4es in the sub-scanning direction for each color is expressed by:
xcex4esY=v*{(tsf2xe2x88x92tsf1)+(tsr2xe2x88x92tsr1)}/2xe2x88x92dsYxe2x80x83xe2x80x83(1)
xcex4esM=v*{(tsf3xe2x88x92tsf1)+(tsr3xe2x88x92tsr1)}/2xe2x88x92dsMxe2x80x83xe2x80x83(2)
xcex4esC=v*{(tsf4xe2x88x92tsf1)+(tsr4xe2x88x92tsr1)}/2xe2x88x92dsCxe2x80x83xe2x80x83(3)
Further, left and right aberrations xcex4emf and xcex4emr in the main-scanning direction for each color are expressed by the following (12) to (17) on the basis of the following (4) to (11).
dmfBk=v*(tmf1xe2x88x92tsf1)xe2x80x83xe2x80x83(4)
dmfY=v*(tmf2xe2x88x92tsf2)xe2x80x83xe2x80x83(5)
dmfM=v*(tmf3xe2x88x92tsf3)xe2x80x83xe2x80x83(6)
dmfC=v*(tmf4xe2x88x92tsf4)xe2x80x83xe2x80x83(7)
dmrBK=v*(tmf1xe2x88x92tsf1)xe2x80x83xe2x80x83(8)
dmrY=v*(tmf2xe2x88x92tsf2)xe2x80x83xe2x80x83(9)
dmrM=v*(tmf3xe2x88x92tsf3)xe2x80x83xe2x80x83(10)
dmrC=v*(tmf4xe2x88x92tsf4)xe2x80x83xe2x80x83(11)
xcex4emfY=dmfYxe2x88x92dmfBKxe2x80x83xe2x80x83(12)
xcex4emfM=dmfMxe2x88x92dmfBKxe2x80x83xe2x80x83(13)
xcex4emfC=dmfCxe2x88x92dmfBKxe2x80x83xe2x80x83(14)
xe2x80x83xcex4emrY=dmrYxe2x88x92dmrBKxe2x80x83xe2x80x83(15)
xcex4emrM=dmrMxe2x88x92dmrBKxe2x80x83xe2x80x83(16)
xcex4emrC=dmrCxe2x88x92dmrBKxe2x80x83xe2x80x83(17)
Thus, the aberration directions can be determined depending on whether the results of the calculations are positive or negative. The xcex4emf is used to correct the write start position, whereas the xcex4emrxe2x88x92xcex4emf is used to correct the scanning width.
In the case when the scanning width has an error, the write start position is calculated by using not only the xcex4emf but also the variation of the image frequency associated with the correction of the scanning width.
In the following description, if the color aberration detecting patterns in FIG. 3 are used, the line width thereof is assumed to be 35 dots, the length thereof in the main-scanning direction is assumed to be 100 dots, and the space between the patterns is assumed to correspond to 50 dots.
The conventional examples, however, have the following disadvantages.
Due to the eccentricity of a belt driving roller, the irregular rotation of a driving part, or the like, the movement speed v mm/s of the conveying belt 3 is not always fixed, resulting in a detection error proportional to the temporal difference between the patterns.
If this detection error is caused by periodically irregular driving, it can be eliminated by arranging the plural sets of position aberration detecting patterns at appropriate locations, calculating the misalignments thereof, and averaging them. For non-periodic irregularity, however, the detection error cannot be eliminated even with the averaging process.
For example, with a 600-dpi image forming apparatus in which one dot is 42.3 xcexcm, if the position aberration is to be detected with an accuracy of xc2xc to xe2x85x9 dots, detection errors arising from non-periodic irregularity are not negligible. Thus, action must be taken to avoid being affected by the non-periodic irregularity.
Specifically, for this purpose, the interval between the alignment detecting pattern for the reference color and the alignment detecting pattern for the detected color must be reduced, and the alignment detecting patterns must be closely transferred onto the conveying belt.
The present invention is adapted to solve the above problems, and it is an object thereof to reduce errors in position aberration detection arising from non-periodic irregularity in order to enable very accurate position aberration corrections.
The present invention is characterized by an image forming apparatus, including:
a plurality of image forming means each having an image bearing body and write means for forming images of different colors on the image bearing body;
a moving member moving so as to transfer the images formed on the image bearing body by the plurality of image forming means to corresponding transfer locations;
a pattern forming means for controlling the plurality of image forming means so as to form a plurality of pattern images in a direction crossing a movement direction of the moving member so that two pattern images of a predetermined reference color are transferred onto the moving member with one pattern image of a different color transferred therebetween;
a pattern detecting means for detecting the pattern images transferred onto the moving member;
an aberration detecting means for detecting aberration of the image of the different color with respect to the images of the reference color on the basis of an output from the pattern detecting means; and
a correction means for correcting the aberration of the image of the different color on the basis of aberration detected by the aberration detecting means.
Furthermore, the present invention is characterized by an image forming apparatus, comprising:
a plurality of image forming means each having an image bearing body and write means for forming images of different colors on the image bearing body;
a moving member moving so as to transfer the images formed on the image bearing body by the plurality of image forming means to corresponding transfer locations;
a pattern forming means for controlling the plurality of image forming means so as to form a plurality of pattern images in a direction crossing a movement direction of the moving member so that a pattern image of a predetermined reference color and a pattern image of a different color are alternatively transferred onto the moving member a predetermined number of times;
a pattern detecting means for detecting the pattern images transferred onto said moving member;
an aberration detecting means for detecting aberration of the image of the different color with respect to the images of the reference color on the basis of an output from the pattern detecting means; and
a correction means for correcting the aberration of the image of the different color on the basis of the aberration detected by the aberration detecting means.